Chip type LEDs in which elements are directly packaged on a pattern of a printed wiring substrate and are encapsulated with resins, are advantageous for miniaturization and thickness reduction, and therefore, such chip type LEDs are widely used in electronic instruments such as number pad illumination of mobile telephones, and backlights for small-sized liquid crystal displays.
In recent years, the technologies for increasing the luminance of LEDs are in marked progress, and the luminance of LEDs is ever increasing. Along with this progress, the amount of heat emission of the LED elements themselves has increased, and the thermal load exerted on printed wiring substrates is increasing, so that the temperature around the LED elements may rise above 100° C. in some cases. Furthermore, in a process of producing a substrate for mounting LED, a thermosetting treatment of encapsulating resins or employment of lead (Pb)-free solder is carried out, so that the substrates may be exposed to a temperature of about 260° C. to 300° C. in a reflow process. Thus, substrates for mounting LED are more exposed to thermal environments at high temperatures.
In printed wiring substrates formed from thermosetting resin compositions that have been conventionally used, there may be observed a tendency that the degree of whiteness decreases, causing yellowing, and the reflection efficiency is deteriorated, in such an environment under thermal load. Thus, there is still room for improvement in those substrates for mounting next-generation high-luminance LEDs of the future.
On the other hand, ceramic substrates are excellent in terms of heat resistance, but have a rigid and brittle nature. Therefore, there are limitations in attempting to promote size increment and thickness reduction as in the case of resin substrates, and there is a possibility that it may be difficult to deal with ceramic substrates as substrates for general applications for illumination or display applications in the future.
In regard to such problems, for example, the invention described in Patent Document 1 and the like have suggested light reflecting bodies having excellent heat resistance and light resistance, which are formed from a silicone resin or a silicone rubber and an inorganic filler, and have a specular reflectance of 88% or greater.